Format of EIGRP Packet

EIGRP Packet format is as below:-

Protocol number 88 is used to carry it straight in IP packets.
The longest EIGRP packet can be is determined by the interface’s highest IP MTU. This is usually 1500 bytes for the whole IP packet, leaving 1480 bytes for the EIGRP packet.
Each EIGRP packet has a 20-byte header & TLVs.
There is no RTP header that is easy to see.
Most of the RTP features (along with certain TLVs) are instead found in the Flags, Sequence Number, and Acknowledgement number fields.

Version: Version is a four-bit parameter that is used to indicate the protocol version of the EIGRP process that originated on the network. It is set to 2, and has not changed since it was first released.
Opcode: 4-bit field that specifies the EIGRP packet type. 1 = Update, 3 = Query, 4 = Reply, 5 = Hello/Ack, 10 = SIA-Query, 11 = SIA-Reply.
Checksum: A 24-bit field that is utilized, without the IP header, to perform a sanity check on the EIGRP packet.
Flags: The 32-bit field that indicates particular flags is called a flag. 0x1 is the Init indicator, which is used during the initial adjacency buildup. 0x2 is the Conditional Receive indicator, which is used by RTP to allow this message to be received only by a subset of receivers. 0x4 is the Restart indicator, which indicates that a router has restarted. 0x8 is the End-of-Table indicator, which indicates that the transmission of the entire EIGRP database has been completed.
Sequence: The sequence field is 32 bits long and comprises a number that RTP uses to ensure the sequential delivery of trustworthy packets.
Acknowledgement: This is a 32-bit field used by RTP that holds the sequence number of the last packet received from the neighbor. A Hello message that has an ACK field that is not zero is not a Hello. It is an ACK. There are never more than one acknowledgement.
Virtual Router ID: This is a 16-bit field that tells you which virtual router this packet is linked to. It’s 0x1 for a Unicast Address Family, 0x2 for a Multicast Address Family, and 0x8000 for a Unicast Service Address Family.
Autonomous System Number: This is a 16-bit field that tells you the EIGRP domain number.
Type-Length-Value: This is a field that holds route records and gives EIGRP DUAL information.

Several kinds:

0x0001 EIGRP Parameters ( General TLV Types )

0x0002 Authentication Type ( General TLV Types )

0x0003 Sequence ( General TLV Types )

0x0004 Software Version ( General TLV Types )

0x0005 Next Multicast Sequence ( General TLV Types )

0x0102 IPv4 Internal Routes ( IP-Specific TLV Types )

0x0103 IPv4 External Routes ( IP-Specific TLV Types )

0x0402 IPv6 Internal Routes ( IP-Specific TLV Types )

0x0403 IPv6 External Routes ( IP-Specific TLV Types )

0x0602 Multi Protocol Internal Routes ( AFI-Specific TLV Types )

0x0603 Multi Protocol External Routes ( AFI-Specific TLV Types )

The EIGRP Packets

The following seven types of packets are used by EIGRP:

Hello

Acknowledgement

Update (RTP)

Query (RTP)

Reply (RTP)

SIA-Query (RTP)

SIA-Reply (RTP)

Because EIGRP guarantees the dependable delivery and correct order of all RTP marked packets, they are also known as reliable packets.The show ip eigrp traffic command can be used to get statistics on all the EIGRP packets, both sent and received.

Hello

Once EIGRP is turned on for a link, periodic “Hello” messages are sent.
Used to find neighbors, make sure they have the same configuration (a shared IP subnet, the same AS number, K values, and optional authentication), and keep the connection open between neighbors.
It was sent to 224.0.0.10 in IPv4 and FF02::A in IPv6.
In the event that static neighbors are set up, hello packets are sent as single messages to the neighbors’ configured address.
The Hello timeout is set to 5 seconds by default.
NBMA with a speed of 1544 kbps or less has a 60-second Hello interval by default.
The opcode is 5, and it is not acknowledged.

Acknowledgement

In order to ensure the reliable delivery of EIGRP packets, the ACK packet is utilized to confirm receipt of certain packets.
Ack packets are sent in response to Update, SIA-Query, Reply, and SIA-Reply.
This is unicasted to the sender of the acknowledged packet.
The basic structure of an ACK packet is a Hello packet without any TLVs in its body and an Acknowledgement number field that is not zero and set to the Sequence number of the reliable packet being acknowledged.
Consistent with the Hello packet, with same Opcode of 5.
It is unnecessary to send a separate ACK if EIGRP can insert the acknowledgment number into the Acknowledgment number field of the reliable packet, even if both are waiting to be forwarded to the same neighbor.
Acknowledgment numbers can only be carried by unicast packets.

Update

conveys the reachability of destinations and contains updates to route information.
Can be both unicasted & multicasted.
There are some Rules for the updated which are as below:-

During the process of a new adjacency buildup, update packets are uniquely transmitted between the neighbors that have recently been discovered. EIGRP may decide to use multicasts to synchronize neighbors in certain circumstances, such as when it connects to a DMVPN network. Only the effectiveness of the initial synchronization procedure is affected by the option; the actual contents of the information that is being transferred are not affected in any way by the choice.
It is only when the routers have completely synchronized that additional updates are transmitted as multicasts.
In the event that a neighbor does not acknowledge the arrival of an Update packet, EIGRP will resend the Update packet to the neighbor who did not respond to the initial transmission as a unicast.
Unicast is always used by EIGRP, not only for point-to-point connections but also for neighbors that are statically configured.

Delivered reliably – It is always acknowledged and sent again if no response is heard.
Opcode of 1.

Query

used to assign the responsibility of figuring out the best path to a destination to neighbors.
Delivered reliably.
Both unicasted and multicasted.
Queries are transmitted as multicasts by default on multiaccess interfaces with only dynamic neighbors.
If recognized in time, a Query is sent again to the neighbor that didn’t respond as a unicast message.
Queries are always sent as unicasts on point-to-point connections and to neighbors that are set up in a static way.
Although an ACK is required for every Query that has been received, this ACK does not mean that the Query has been answered; it just means that it has been received.
The reply is used for that pupose.
Opcode of 3.

Reply

Query packets that are sent back carry the sender’s current distance to the target, which takes into account the change in topology that caused the Query.
Always sent only to the originator of the Query message and always delivered regularly.
OpCode of 4.

SIA-Query and SIA-Reply

Used to check if a neighbor is reachable and still working on the corresponding diffusing computation throughout a protracted diffusing computation. This neighbor has not sent a Reply to a Query yet.
To verify if a neighbor is still processing the initial Query, you can use the SIA-Query.
An SIA-Reply will be sent out instantly by an accessible neighbor that is still processing diffused data.
A reset is applied to the maximum diffusing computation timer.
The SIA-Query and SIA-Reply protocols are both trustworthy and unicast.
The Opcode used by SIA-Query is 10.
An Opcode of 11 is utilized by SIA-Reply.

Reliable Transport Protocol

In charge of sending and receiving EIGRP packets.
Packets like Update, Query, Reply, SIA-Query, and SIA-Reply (both unicast and multicast) are reliably sent.

In the packet body, there is a Sequence number that is not zero.
The Sequence number is a global value that is kept for each EIGRP process instance on a router. It goes up every time one of these packets is sent from an EIGRP-enabled port by the instance, no matter which one.
The Sequence number of hello and ack packets (which are not dependable) is set to zero, and they do not make the global Sequence number go up.
Conditional Receive: EIGRP divides its neighbors into two groups: 1) “well-behaved” neighbors that have acknowledged all multicast messages sent so far; and 2) “lagging” routers that have not acknowledged at least one EIGRP packet.
If EIGRP wants to run in parallel, it must include a flag that says “this packet is only for those routers who have received all multicast packets so far” in each in-order multicast packet.
Use of two distinct TLVs—the Sequence TLV and the Next Multicast Sequence TLV—to implement a sequenced hello.
The Upcoming Multicast Order A trustworthy multicast message’s next sequence number is stored in the TLV.
The Sequence TLV basically says “whoever finds himself in this list, ignore the next multicast message with the indicated sequence number” by containing a list of all lagging neighbors by IP address.
A neighbor that gets the Sequenced Hello but isn’t in the Sequence TLV will enter what is known as Conditional Receive mode (CR-mode) since it expects to receive the next multicast packet.
Set the CR flag before sending the next multicast packet.
The packet will be processed as usual by routers operating in CR mode, after which they will quit the CR mode. Other routers will disregard the packet.
As a consequence of this, the router is able to continue using multicast with those routers who do not have any problems receiving and recognizing it, while simultaneously ensuring that the neighbors that are falling behind will not process the multicasts until they are able to catch up.
The multicast flow timer is responsible for determining the amount of time that must pass before acknowledging a neighbor as being behind schedule and transitioning from multicast to unicast communication.

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